1. Field of the Invention
The present invention relates to a recursive filter for use in reducing image noise in an image display technique and an image display apparatus having the recursive filter.
2. Description of the Background Art
In a conventional image display apparatus such as an X-ray apparatus where an X-ray is irradiated to an object to display an objection profile of a moving image of the object on a monitor, a recursive filter for reducing image noise is used by taking advantage of the afterimage effect.
However, when a large motion part of an object is processed by using a recursive filter, the afterimage effect largely affects to reduce image contrast, and an artifact is produced to bring about a deteioration of the image quality. In order to solve such a problem, a conventional recursive filter has been proposed, as disclosed in Japanese Utility Model Laid-Open specification No. 63-55400.
In FIG. 1, there is shown a conventional recursive filter for use in an image display apparatus. An analog video signal output by an image intensifier (I.I.) (not shown) is fed to an A/D (analog-digital) converter 1 where the analog video signal is converted into a first digital video signal. Then, the first digital video signal is sent from the A/D converter 1 to a frame memory 4 through a selector 6 having a switch therein and is once stored in the frame memory 4. The digital video signal (hereinafter referred to as the second digital video signal) read out of the frame memory 4 is then fed to a D/A (digital-angalog) converter 8 where the second digital video signal is converted into an analog video signal, and the analog video signal is fed to a monitor for displaying an image thereon. The first digital video signal also sent from the A/D converter 1 to a comparator 7 in which the first digital video signal is compared with the second digital video signal read out of the frame memory 4 evey corresponding pixel.
For example, when a difference between the first and second digital video signals is larger than a certain threshold value, the comparator 7 changes over the switch from the terminal A to the terminal B in the selector 6. Hence, the first digital video signal is passed to the frame memory 4 through the selector 6 and is stored in the frame memory 4 and is fed to the monitor to display the image thereon as well.
In turn, when the difference between the first and second digital video signals is smaller than the threshold value, the switch of th selector 6 is changed over from the terminal B to the terminal A by the comparator 7. Then, the first digital video signal is fed from the A/D converter 1 to a multiplier 2 where the first digital video signal is multiplied by a constant K (0 to 1), and the first digital video signal multiplied by K is sent to an adder 3. In another multiplier 5, the second digital video signal fed from the frame memory 4 is multiplied by another constant (1-K), and the second digital video signal multiplied by (1-K) is also sent to the adder 3. In the adder 3, the two digital video signals are added to obtain a new digital video signal as new image data which is fed to the frame memory 4 and is once stored therein and is also fed to the monitor to display the image thereon as well. This processs is carried out every corresponding pixel.
In the conventional recursive filter, the improvement can be performed in reducing the image noise. However, whether or not the recursive filter process is carried out is determined by the threshold value, and an unnatural feeling remains around the threshold value. Hence, the variation around the threshold value is not properly reproduced on the display, and the variation is not smooth around the threshold value.
Further, in the conventional recursive filter, it is difficult to determine the threshold value. That is, when the threshold value is too small, the image noise is apt to be produced, and, in turn, when the threshold value is too large, the image contrast is reduced, and the artifact is produced.